A recombinant vector containing the normal human erbB-2 cDNA was generated to determine whether this growth factor receptor- like gene could transform in the NIH/3T3 transfection assay. erbB-2 was shown to be a potent oncogene when overexpressed in NIH/3T3 cells. These findings demonstrate a new mechanism for acquisition of oncogenic properties by genes encoding growth factor receptor-like proteins and provide a functional basis for the role of their overexpression in the development of human malignancies. The interactions of murine mast cell lines with B-cell stimulatory factor-1 (BSF-1/IL-4) were explored. BSF-1 mRNA was expressed by a majority of transformed mast cell lines and by five IL-3- dependent mast cell lines. BSF-1 activity was detected in the supernatants of transformed mast cells. The role of BSF-1 as a mast cell growth factor and its constitutive production by transformed mast cells raises the possibility that BSF-1 may act as an autocrine growth factor for some transformed mast cells. Furthermore, production of BSF-1 mRNA by non-transformed cells indicates mast cells may be an important physiologic source of this factor. The arrangement of immunoglobulin genes was examined in a series of lymphoid cell lines transformed with Harvey murine sarcoma virus in vitro. One fetal liver transformant was shown to possess a germline configuration for the immunoglobulin gene family. This line was shown to frequently rearrange either immunoglobulin on T-cell receptor genes during subcloning. Therefore, this transformant appears to represent the earliest stage in lymphoid development.